1. Field of Invention
This invention relates generally to a drawbridge apparatus and more particularly to a drawbridge with an initial vertical motion followed by a sliding or horizontal-linear motion.
2. Description of Prior Art
Drawbridge designs based on vertical lift, vertical rotation and horizontal rotational movement have often been utilized as drawbridges for pedestrian, vehicular, and railroad traffic. There are various vertical lift bridge designs in the prior art, which use vertical movement as their only means of creating the reversible draw. Several types of drawbridge designs are in the prior art based on the cantilever including a bascule (only vertical rotation), a swing (only horizontal rotation) as their only means of creating the reversible draw. Sliding (only horizontal-linear movement) bridges for creating the reversible draw are in the prior art but have not come into use.
Of these four mentioned drawbridge types, each is based on a single direction of movement to create the draw. Only the vertical lift, bascule and swing bridges have come into practical use. Although drawbridges based on the sliding deck movement have many advantages, the use of the sliding bridge movement has not come into use due to the unobvious solution to the problem of the adjacent bridge approaches obstructing the sliding movement of the bridge. Whereas the vertical lift, bascule and swing bridges have an obvious simple connection between the bridge and the adjacent approaches that does not interfere with the movement of the bridge deck.
Vertical Lift Bridge in Prior Art
The draw or open position for the vertical lift bridge is created when the deck of the structure is moved vertically to a sufficient height to allow for marine (or other) traffic. The draw is the opening created when the bridge deck is removed. The clear-height is the height to which traffic can safely pass under the open vertical lift bridge. The required clear-height is usually determined from the records of past use of the channel. Therefore, the vertical clearance of all vertical lift bridges is finite and after the structure is built only traffic whose overall height is lower than the vertical lift bridges constructed clear-height will be able to traverse the channel. But since the actual clear-height will vary with the elevation of the water in the channel, marine vessels must always be aware of the changing water elevations since at times a particular vessel may not be able to safely pass under the vertical lift bridge's draw.
In addition, if the proposed vertical lift bridge structure must accommodate several lanes of traffic the wide deck and resultant heavy loads require the vertical lift bridge's superstructure to become quite massive. Also as the required clear-height of the channel increases the height of the vertical lift bridge superstructure increases. This height (perhaps fifty (50) or more feet above the adjacent ground elevation) of the vertical lift bridge superstructure poses risk to life and limb during construction as well as during future vertical lift bridge maintenance. As a result vertical lift spans can and have become quite expensive to design, construct and maintain.
The following patents by L. H. Shoemaker (U.S. Pat. No. 2,482,562) and T. Sakamoto (U.S. Pat. No. 2,040,445) are examples of vertical lift bridges in the prior art.
Bascular Bridge in Prior Art
For the bascular type cantilever bridge, the bridge deck is moved to create the draw by rotating the structure vertically (usually 70 to 80 degrees from horizontal) to open sufficiently for boats to pass and then to close again. For the bascular bridge a counterbalanced weight is necessary to facilitate the lifting of the heavy deck structure. For the bascular bridge, a major design weakness is the need to design the clear-span much wider than the clear-span required by marine (or other) traffic. The clear-span is the transverse clearance required by marine (or other) traffic to safely traverse an open drawbridge. The additional clear-span width is required in the design to safely offset the bascular bridge deck to prevent any marine collisions with said bascular bridge deck.
In the case of a twin-cantilevered bascule bridge the two sides of the bridge project inwardly and are essentially two simple cantilevered beams. Heavy loads at the point where the twin-cantilevers touch at the center of the bridge must be accounted for in the design resulting in heavy bascule cantilever structural members.
Therefore the typical bascular bridge's cantilever length is normally much longer than necessary for the required clear-span of marine (or other) traffic and the heavy-duty construction required to support the simple twin-cantilevered beams and corresponding added weight to the entire structure makes the bascular bridge structure a very cumbersome device to move quickly. Thus vehicular traffic is typically tied up waiting long periods for the bridge to open sufficiently for boats to pass and then to slowly close again. Also when overcoming the above cited design weaknesses by designing the typical bascular bridge larger than would otherwise be needed by vehicular traffic, the financial expense and time required to construct the bridge structure increases substantially.
In addition, when the proposed bridge structure must accommodate several lanes of traffic the resulting added width of the bascular bridge can become difficult and expensive to design and construct making their design and construction impractical for many proposed bridge locations. Also it is common for the bascular bridge to be closed to vehicular traffic for several days while in maintenance.
The patent to Patten (U.S. Pat. No. 5,421,051) discusses attempts at solving some of the bascule bridge design problems.
The following patents by Lucian I. Nedelcu (U.S. Pat. No. 4,751,758) and Ivan Dvorak, Shankar Nair and Vinod C. Patel (U.S. Pat. No. 5,454,127) are examples of bascular bridges in the prior art.
Swing Bridge in Prior Art
For the swing type cantilever bridge the draw is created by rotating the bridge deck horizontally approximately 90 degrees to open sufficiently for marine (or other) traffic to pass and then to reverse the direction of the rotation to close the draw. The typical swing bridge superstructure is supported at its center on a turntable creating a two-armed cantilever bridge deck balanced on said turntable.
The swing bridge's cantilever swing span is typically offset to allow for the required clear-width of marine traffic and to safely place the swing bridge's structure where minimal damage to the bridge would occur in the case of collisions. Also the wider the swing bridge becomes (due to additional lanes of traffic) a proportional increasing of the offset is required. As a result the swing span bridge cantilevers are typically much longer than required to maintain marine traffic.
If the swing span bridge is to acceptably perform without a “see-saw” effect on the traveling public the long cantilevers must be designed with heavy members. This also adds significant weight to the typical swing drawbridge requiring the bridge to have a much heavier construction than is otherwise needed to support design loads. Therefore, the typical horizontal swing bridge's cantilever length is normally much longer than is required to effectively create a clear-span for marine (or other) traffic. The heavy-duty construction required for the bridge to provide an acceptable “ride” performance for the traveling public and corresponding added weight to the entire structure makes the swing bridge a very cumbersome device to move quickly. Thus traffic is typically tied up waiting long periods for the bridge to open sufficiently for boats to pass and then to slowly close again.
In addition, if the proposed bridge structure must accommodate several lanes of traffic, the clear-space requirements for the bridges swing motion along with the above cited horizontal swing bridge disadvantages can make it difficult and expensive to design and construct. Also in many cases the swing type cantilever bridge turntable is placed in the channel, making inspection and maintenance of the main support structure difficult and expensive. Also it is common for the swing bridge to be closed to vehicular traffic for several days while in maintenance.
The patent to J. B. Strauss (U.S. Pat. No. 1,158,084) is an example of a horizontal swing span in the prior art.
Slide Bridge in Prior Art
For the slide type cantilever bridge the draw is created by sliding the bridge deck horizontally to allow for boats (or other) traffic to pass. The slide direction is then reversed to close the draw. The typical slide type cantilever bridge superstructure is supported at one end creating a cantilever bridge deck.
The sliding cantilever bridge design, although in prior art, has not come into common use likely due to the cumbersome and impractical solutions to the problem of the adjacent bridge approaches obstructing the sliding motion of the said cantilever bridge. One solution was to have a low approach to the bridge with a ramp attached to the bridge deck that would slide with the said bridge deck. The ramp solution is not practical for modern high speed traffic since the ramp may need to be several hundred feet long. Another solution was to have a removable deck which would be removed by some means prior to the sliding of the bridge deck. The removable deck solution is also not practical because it is in essence a second drawbridge that has to be constructed adjacent to a sliding drawbridge.
The patents to W. E. Aston (U.S. Pat. No. 567,875) and T. R. Bevans (U.S. Pat. No. 663,484) are examples of slide type cantilever bridges in the prior art.
Summary of Prior Art
In consideration of the limitations of the drawbridge structures as disclosed in the prior art and as discussed and cited above, it should be apparent that an effective solution to the problem of quickly and efficiently reversibly spanning a distance without having to build the drawbridge structure to accommodate above cited weaknesses is needed. Accordingly, the present invention of a lift-slide drawbridge provides significant advantages over previous drawbridge structures used to reversibly span over waterways or other obstacles.